The present invention generally relates to air separation module (ASM) fiber material and methods for preparing ASM fiber material and, more particularly, to ASM fiber material formulated using a polycarboxylic acid in a polymer casting solution.
ASMs are a key component for the nitrogen generation systems (NGS) needed to provide fuel tank inerting for commercial and military aircraft. Useful membranes for separating oxygen from nitrogen must have sufficient selectivity to distinguish between these two similar gases, and must also have high flux. Hollow fiber membranes are often used as ASMs.
Hollow fiber membranes are generally made from solution spinning, which introduces a large amount of solvent into the middle “bore” of the hollow fiber. This high concentration of solvent, along with the solvent already present in the polymer casting solution, may yield large void spaces in the walls of the fiber. These void spaces may weaken the structure of the hollow fiber and lower the pressure rating of the fiber.
While several types of hollow fiber membrane morphologies exist, it is believed that a hollow fiber with a gradient porosity (and effectively, gradient density) reduces the risk of initial defects, as well as defects exposed during operation. A defect in the context of a hollow fiber membrane (HFM) may be defined as an opening or path through the dense selective layer of the HFM through which both permeate and retentate matter pass with lessened, little or no restriction. This causes the overall selectivity of the membrane to decrease, and with enough (or severe enough) occurrences of defects, the membrane may be rendered useless.
When the morphology of the HFM is controlled via formulation and/or processing, the chance of random defects causing loss of performance or failure is lowered significantly. To form a HFM with gradient porosity requires the rapid removal of solvent from the fiber wall during the HFM production process.
Referring to FIGS. 1 and 2, there are shown cross-sectional views of a hollow fiber membrane formed by conventional methods. A hollow fiber membrane 100 was prepared using 28% of a polyimide in a polymer casting solution consisting of NMP, IPA, acetone and dioxolane. The fiber membrane 100 was spun into a room temperature water bath and drawn onto a take-up roller.
Referring to FIGS. 3 and 4, there are shown cross-sectional views of a second hollow fiber membrane formed by conventional methods. A second hollow fiber membrane 105 was prepared using 28% of the same polyimide of FIGS. 1 and 2 in a polymer casting solution consisting of NMP, IPA, acetone and dioxolane. The fiber membrane 105 was spun into a room temperature water bath and drawn onto a take-up roller.
In both of these hollow fiber membranes 100, 105, void spaces 110 can be seen in the wall of the hollow fiber membranes 100, 105. As can be seen from comparing FIGS. 2 and 4, the frequency (per volume fiber) and size of these void spaces 110 may vary between batches made of similar materials.
As can be seen, there is a need for a hollow fiber membrane formulation and methods for producing hollow fiber membranes that may reduce or eliminate the void spaces in the walls of the fibers.